For protection of devices in a circuit of a switching regulator or operation control of a load on the secondary side, a voltage value of a commercial power supply and a current value inputted to the switching regulator, and a current value outputted from the switching regulator are detected. As a method for detecting the commercial power supply voltage value and the current value inputted to the switching regulator, a method using a photocoupler and a current transformer is proposed (See Japanese Patent Application Laid-Open No. Hei 10-274901).
The above conventional method will be described with reference to FIG. 11.
An alternating current supplied from a commercial power supply 1 is rectified and smoothed by a rectifier bridge 2 and a primary smoothing condenser 3, to an approximately constant voltage Vp. The voltage Vp is supplied via a transformer 5 to an FET 6. When switching of the FET 6 is caused by a driving circuit 4, a pulse voltage is induced on the secondary side of the transformer 5. The induced pulse voltage is rectified and smoothed by a secondary rectifier diode 7 and a secondary smoothing condenser 8, to a predetermined voltage Vout. The voltage Vout is supplied to a load 9.
Next, a method for detecting a voltage value of the commercial power supply 1 in the switching regulator will be described.
The primary voltage Vp is divided with resistors 17 and 180, and supplied to an LED of a photocoupler 13. As the amount of light emission from the LED of the photocoupler 13 is proportional to the voltage Vp, a photo transistor corrector current in the photocoupler 13 is also proportional to the voltage Vp. Accordingly, a voltage value Vvp supplied to a calculation unit 10 is proportional to the primary voltage value Vp. The calculation unit 10 obtains the primary voltage value Vp by inverse calculation from resistance values of the resistors 17 and 180 and a photo-electric current transfer ratio (CTR value) of the photocoupler 13, thereby obtains a voltage value Vin of the commercial power supply 1.
Next, a method for detecting a current value inputted to the switching regulator will be described.
A primary terminal of a current transformer 11 is connected to an input unit of the switching regulator. Accordingly, a current proportional to an input AC current value Iin flows in a secondary side terminal of the current transformer 11. The current is converted by a resistor 12 to a voltage, and supplied to a differential amplifier 14. An output voltage Viin from the differential amplifier 14 is smoothed by a resistor 16 and a condenser 190 and supplied to the calculation unit 10. The calculation unit 10 calculates a current value Iin inputted into the switching regulator by inverse operation from the ratio of winding of the current transformer 11 and the resistance value of the resistor 12, based on the voltage Viin.
Further, as a method for detecting a current value outputted from a switching regulator, use of current detection resistor is proposed (See Japanese Patent Application Laid-Open No. Hei 05-076173).
Next, a method for detecting a current value outputted from a switching regulator will be described with reference to FIG. 12.
A current detection resistor 340 is connected in series with an output of the switching regulator. A voltage Viout proportional to an output current Iout from the switching regulator occurs at between the both ends of the current detection resistor 340. The voltage Viout is detected by a differential amplifier 33, and supplied to the calculation unit 10. The calculation unit 10 converts the voltage Viout into a digital signal, and obtains an output current Iout outputted from the switching regulator by inverse operation (Viout/R) from a resistance value (R) of the current detection resistor 340.
Further, a control in a case where the switching regulator in FIG. 11 is incorporated in an image forming apparatus (e.g., a laser-beam printer) will be described with reference to FIG. 13.
In FIG. 13, the commercial power supply 1 is supplied to the switching regulator shown in FIG. 11 and a fixation power supply 34, and an actuator 36 in place of the load 9 in FIG. 11 is connected to the secondary side of the transformer 5.
The commercial power supply 1 is supplied to the above switching regulator and also supplied to the fixation power supply 34. An output from the fixation power supply 34 is supplied to a fixation unit 37. The fixation unit 37 melts a toner image and fixes the image to the surface of print sheet. The calculation unit 10 turns on/off output power from the fixation power supply 34 to the fixation unit 37, based on a temperature information signal thm supplied from a temperature detection unit (not shown) provided in the fixation unit 37, such that the fixation unit 37 has an approximately constant temperature. At this time, the timing of on/off in the fixation power supply 34 is regulated by an on/off timing signal (ON/OFF) supplied from the calculation unit 10 to the fixation power supply 34. Further, the output power from the fixation power supply 34 is regulated by a power upper limit signal Pwtgt.
Generally, a current value consumable by an electrical equipment from the commercial power supply 1 is regulated with a maximum current value Imax by the safety standard. For example, in Japan, the current value consumable by electrical equipment from a commercial outlet is up to 15 A. Accordingly, the calculation unit 10 sequentially calculates the power upper limit signal Pwtgt such that the current inputted to the image forming apparatus from the commercial power supply 1 does not exceed the regulated current value Imax, and controls the power supplied to the fixation unit 37. This operation will be described with reference to FIGS. 14A and 14B.
FIG. 14A depicts the transition of the current Iin inputted into the switching regulator. At timing T1, when the main switch (not shown) of the image forming apparatus is turned on, the calculation unit 10 appropriately operates the actuator 36 in preparation for image forming operation. In accordance with this operation, the current Iin inputted to the switching regulator increases. The calculation unit 10 sequentially detects the input current value Iin and calculates the difference from the regulated current value Imax, Itgt=Imax−Iin. That is, the difference Itgt is a current value allowable to the fixation unit 37. Further, the calculation unit 10 detects the input voltage Vin, and calculates power allowable to the fixation unit 37, Pwtgt=Itgt×Vin from the current value Itgt and the input voltage Vin.
As shown in FIG. 14B, the calculation unit 10 on/off controls the power Pwtgt inputted into the fixation unit 37 such that the temperature of the fixation unit 37 is approximately constant. In this arrangement, the current inputted to the image forming apparatus from the commercial power supply 1 is controlled not to exceed the regulated current value Imax.
However, the above-described conventional art has following problems. First, for detection of the voltage value and current value of the commercial power supply 1 inputted into the switching regulator, primary-secondary insulating parts such as the photocoupler 13 and the current transformer 11 are required, and the cost is increased. Secondly, when the current value outputted from the switching regulator is detected, as the current detection resistor 340 is inserted in an output power supply line, electric power loss is caused in the current detection resistor 340. Thirdly, the voltage drop caused in the current detection resistor 340 degrades accuracy of output voltage from the switching regulator.
Lastly, in a case where this conventional switching regulator is incorporated in an image forming apparatus, the primary-secondary insulating parts such as the photocoupler 13 and the current transformer 11 are required and the cost is increased. To observe the regulated current value of commercial power supply without such photocoupler and current transformer, proposed is an image forming apparatus in which a maximum current value consumed by the actuator 36, Iamax, is stored in the calculation unit 10 and power, up to (Imax−Iamax)×Vinmin (Vinmin is a minimum value of commercial power supply voltage), is supplied to the fixation unit 37. However, in this construction, when the current consumed by the actuator 36 is less than Iamax or when the commercial power supply voltage is higher than Vinmin, a current which can be sufficiently supplied from the commercial power supply 1 cannot be efficiently used. This construction increases time from power-on of the main switch before the temperature of the fixation unit 37 has increased to a temperature for image forming operation (warming up time).